Follow these steps to add and configure a Post Grid using the Guten Post Layout plugin:

1. Log in to the WordPress Dashboard as an administrator.
2. Click the Pages menu.
3. Click the Add New option.
4. Click the Add Block button.
5. Click Post Layout from the Guten Post Layout tab to see the following screen, which contains available posts. You will see the posts with or without featured images based on how you created them. If there are no posts on the site, you should add sample posts before adding this block:

6. Click the Layout tab on the right-hand side.
7. Increase the Columns to three from the Layout settings section. Now, the layout will show three columns in grid, apart from the first row. The first row is fixed to two columns. 
8. Click the Publish button.

Now, you can use the View Page link to view the page from the frontend as shown in the following screenshot:

The page will contain a grid of posts with featured image, title, and author information with the default design.